ORTHOGONAL FREQUENCYDIVISION MULTIPLEXINGFOR WIRELESS CHANNELSLEONARD J. CIMINI, JR.YE (GEOFFREY) LIAT&T LABS - RESEARCHORTHOGONAL FREQUENCY DIVISION MULTIPLEXING FOR WIRELESS CHANNELSLeonard J. Cimini, Jr. AT&T Labs – Research 100 Schulz Dr., Rm. 4-146 Red Bank, NJ 07701-7033, USAEmail: [email protected]/Fax: 1-732-345-3129/3039 Ye (Geoffrey) Li AT&T Labs – Research100 Schulz Dr., Rm. 4-152Red Bank, NJ 07701-7033, USAEmail: [email protected]/Fax: 1-732-345-3132/3039ABSTRACTOrthogonal frequency division multiplexing (OFDM) has been shownto be an effective technique to combat multipath fading in wireless communications. It has been successfully used for HF radio applications and has been chosen as the standard for digital audio broadcasting and digital terrestrial TV broadcasting in Europe and high-speed wireless local areas networks. In this tutorial, we present the basic principles of OFDM and discuss the problems, and some of the potential solutions, in implementing an OFDM system. Techniques for peak-to-average power ratio reduction, time and frequency synchronization, and channel estimation will be discussed. We conclude with a brief overview of current application areasBIOGRAPHIES OF PRESENTERS:Leonard J. Cimini, Jr., received the B.S.E. (summa cum laude), M.S.E. and Ph.D. degrees in electrical engineering from the University of Pennsylvania in 1978, 1979, and 1982, respectively. During the graduate work he was supported by a National Science Foundation Fellowship. Since 1982, he has been employed at AT&T, where his research interests are in wireless communications systems. Dr. Ciminiis a member of Tau Beta Pi and Eta Kappa Nu. He has been very active in the IEEE Communications Society and is Editor-in-Chief of the IEEE J-SAC: Wireless Communications Series. He is also an Adjunct Professor at the University of Pennsylvania.Ye (Geoffrey) Li received the B.Eng and M.Eng degrees in 1983 and 1986, respectively, from the Department of Wireless Engineering,Nanjing Institute of Technology, Nanjing, China, and the Ph.D. degree in Electrical Engineering in 1994, Auburn University, Alabama. Since May 1996, he has been with AT&T Labs - Research. His current research interests are in statistical signal processing and wireless communications. He has served as a guest editor for a special issue on Signal Processing for Wireless Communications for the IEEE J-SAC and is an editor for Wireless Communication Theory for the IEEE Transactions on Communications.• Introduction• Basic Concepts• Peak-to-Average Power Ratio Reduction• Time and Frequency Synchronization• Channel Estimation• Applications• Summary• ReferencesOUTLINE• Introduction• Basic Concepts• Peak-to-Average Power Ratio Reduction• Time and Frequency Synchronization• Channel Estimation• Applications• Summary• ReferencesOUTLINEINTRODUCTION• Motivation• Radio Environment • Brief HistoryMOTIVATION• High-bit-rate wireless applications• Limitations caused by the radio environment• OFDM can overcome these inherent bit rate limitationsPATH LOSS MODEL• Path Loss• Shadow Fading• Multipath• Flat fading• Doppler spread• Delay spread• Interferencewhere Pris the local mean received signal PATH LOSS MODEL• Different, often complicated, models are used for different environments. • A simple model for path loss, L, isThe path loss exponent αααα = 2 in free space; 2 ≤≤≤≤αααα≤≤≤≤ 4 in typical environments.power, Ptis the transmitted power, and d is the transmitter receiver distance.αααα========d1KPPLtrSHADOW FADING• The received signal is shadowed by obstructions such as hills and buildings. • This results in variations in the local mean received signal power,• Implications – nonuniform coverage – increases the required transmit power(((()))) (((())))(((()))).dB104,,0N~G whereGdBPdBPS2SSSrr≤≤≤≤σσσσ≤≤≤≤σσσσ++++====MULTIPATHConstructive and destructive interferenceof arriving raysReceived PowerDelay SpreadtdB With Respect to RMS Value00.50.5λλλλ1.5-30-20-101001t, in seconds010 3020x, in wavelength(((())))(((())))ijiitteathi−−−−δδδδ====θθθθ∑∑∑∑FLAT FADING• The delay spread is small compared to thesymbol period. • The received signal envelope, r, follows aRayleigh or Rician distribution.• Implications – increases the required transmit power– causes bursts of errorsshadow fadingRayleigh fadingpath losslog (distance)Received Signal Power (dB)(((())))r log 20SG)dB(rPdBrP ++++++++====DOPPLER SPREAD• A measure of the spectral broadening caused by the channel time variation.• Implications – signal amplitude and phase decorrelateafter a time period ~ 1/fDExample: 900 MHz, 60 mph, fD= 80 Hz5 GHz, 5 mph, fD= 37 Hzλλλλ≤≤≤≤vfDττττlargeTττττsmallT011T2TChannel InputChannel Output0T 2T0T 2TTwo-ray model ττττ = rms delay spread2ττττDelayReceivedPowerττττT• small negligible intersymbol interference • large significant intersymbol interference,which causes an irreducible error floorττττTDELAY SPREADTIME DOMAIN INTERPRETATIONBs= signal bandwidth ≈≈≈≈ 1/TH(f)Bs12ττττf• small flat fadingττττT• large frequency-selective fadingττττTDELAY SPREADFREQUENCY DOMAIN INTERPRETATIONThe rms delay spread imposes a limit on the maximum bit rate. For example, for QPSK• ISI causes an irreducible error floor.ττττ Maximum Bit Rate Mobile (rural) 25 µµµµsec 8 kbps Mobile (city) 2.5 µµµµsec 80 kbps Microcells 500 nsec 400 kbps Large Building 100 nsec 2 Mbps+x+++++xxxxx10-210-410-310-210-110-1100BPSK QPSK OQPSK MSKModulationCoherent DetectionIrreducible PbττττT=rms delay spreadsymbol period BIT RATE LIMITATIONSINTERFRENCE• Frequencies are reused often to maximize spectralefficiency.• For interference-limited systems, the noise floor isdominated by co-channel interference.• Implications – high reuse efficiency requires interferencemitigationBASESTATIONRDαααα====≈≈≈≈++++ RD61ISNIS• Military HF radio (1950’s - 1960’s)– Kineplex– Kathryn• Wireline modem (Telebit, Gandalf)• Cellular modem (Telebit)• Digital audio and terrestrial TV broadcasting (Europe)• Asymmetric digital subscriber line (DMT)• Wireless LANs– IEEE802.11 - National Information Infrastructure– HIPERLAN TYPE